The Würzburg Riese radio telescope with which Leiden astronomers led by Prof. Dr. J.H. Oort in the early 1950s at Radio Kootwijk first mapped the spiral arms in our Milky Way has been found. For a long time, this originally German radar antenna was thought to be lost. A recent quest has shown that this also so-called Kootwijk HI Würzburg has been at Public Observatory Simon Stevin in Hoeven for some time and is now being restored by Museum Deelen to its original military radar function.
It has been almost eighty years since the Utrecht astronomy student Henk van de Hulst, urged on by the Leiden astronomer Prof. Jan Oort, calculated that hydrogen atoms emit very weak radio waves at a wavelength of 21 centimeters. After World War II, the Leiden astronomers and technicians were given access to one of the Würzburg Riese radar antennas from the Atlantic Wall that the PTT had set up at the radio transmitting station Radio Kootwijk (the Netherlands) for various investigations. Led by radio engineer Lex Muller, they converted it into a radio telescope. On March 25, 1951, American researchers Harold Ewen and Edward Purcell of Harvard succeeded in observing the 21-centimeter hydrogen spectral line for the first time. The Dutch astronomers and technicians who tried to do the same thing with the Würzburg at Radio Kootwijk also succeeded on May 11, 1951. After that, systematic research into the structure of the Milky Way commenced in the Netherlands and a first map of the spiral arms of our Milky Way was made (ref. 1). This astronomical success provides Dutch astronomy with much international recognition and lays the foundation for building the 25-meter radio telescope in Dwingeloo for more detailed research of the Milky Way.
After that the Kootwijk HI Würzburg radio telescope made some wanderings and faded into oblivion as a historical scientific instrument. Inquiries and research over the last twenty years have yielded little, and people have become convinced that this Würzburg dish was lost and processed into scrap metal (ref. 2 and 3). And with that, this icon of Dutch radio astronomy would have been permanently lost as a physical monument. At the original location in Radio Kootwijk, only an information board has been placed as a memento (ref. 4 and 5).
There is a lot of photographic material available of Würzburg antennas that had been at the PTT transmitting station in Radio Kootwijk and were later moved to PTT’s NERA receiving station in Nederhorst den Berg (the Netherlands). From that material and from a physical inspection of all kinds of details, Ger Geertsma, an enthusiastic Würzburg researcher, has now shown that the Kootwijk HI Würzburg still exists. He was able to provide proof that the Würzburg dish antenna which had been at Public Observatory Simon Stevin in Hoeven (the Netherlands) for quite some time and came from NERA, is the originally Kootwijk HI Würzburg with which the Dutch astronomers in Radio Kootwijk first mapped our Milky Way.
As an example of the many details that Geertsma provided the proof, two details can be seen in the photo of the Kootwijk HI Würzburg.
1. The two reinforcement ribs at the top center behind the mesh. Those ribs are more common, but not on the other dishes that have been at NERA.
2. The holes around the base of the antenna mount. The two horizontal holes are also more common. However, these have not been found on other dishes in the Netherlands. The two vertical holes (slightly off center) have been added manually and are unique. This detail is highlighted at the bottom left.
These details can also be spotted in the photo of the Würzburg as it was mounted in Hoeven afterwards.
Once Geertsma realized that there were only three radio telescopes at Radio Kootwijk, composed of parts from Würzburg Riesen on the Dutch coast and of home-made parts, all of which went to NERA, he could easily distinguish the three telescopes.
This Würzburg is now at Museum Airbase Deelen, used to be in Hoeven and is the original Kootwijk HI Würzburg. This Würzburg has an early type chassis, a WWII cabin that was replaced in Hoeven with a new cabin in the same style, has an original 7.5-meter dish made of aluminum with four holes around the antenna base and two extra ribs.
And as previously mentioned:
– In themselves the two horizontal holes are not special because many Würzburg Riesen have them. However, these have not been found on other dishes in the Netherlands. The two vertical holes (slightly off center) have been added manually and are unique.
– Also, the two extra ribs are not special as they are more common, but not on the other dishes that have been at NERA. These ribs are utilized in the IFF antennas (IFF = Identification Friend or Foe). These are extra VHF antennas to – in the original military function – keep in touch with the own fighter aircraft in order to distinguish them from enemy aircraft.
This Würzburg is now at the Planetron in Dwingeloo.
This Würzburg has a late type chassis, a PTT-made cabin, and a PTT-made 10-meter dish made of steel tubing and mesh.
This Würzburg is now in the Overloon War Museum.
This Würzburg has an early type chassis, a PTT-made cabin, an original 7.5-meter dish made of aluminum without holes around the antenna base and without two extra ribs.
After a corporate change of Public Observatory Simon Stevin to Public Observatory Quasar and its closure in 2007, the Würzburg from Hoeven went to Museum Airbase Deelen for the restoration to its original military function of a Würzburg radar antenna (type FuMG65) as it was used in the World War II. After the restoration, the Würzburg radar antenna will be exhibited in that museum (ref. 6) and this icon of Dutch radio astronomy will be preserved.
(1) Westerhout, G. (1957). The distribution of atomic hydrogen in the outer parts of the Galactic System (plate B). Bulletin of the Astronomical Institutes of the Netherlands, 13 (475), 201-246. https://articles.adsabs.harvard.edu/full/seri/BAN../0013//0000201.000.html.
(2) Woerden, H. van & Strom, R.G. (2006). The Beginnings of Radio Astronomy in the Netherlands. Journal of Astronomical History and Heritage, 9 (1), 3-20. https://articles.adsabs.harvard.edu//full/2006JAHH….9….3V/0000003.000.html.
(3) Beekman, G. (1999, April). Vijftig jaar Nederlandse radiosterrenkunde: Een verjaardag zonder jarige. Zenit, 26, 154-157. https://www.camras.nl/wp-content/uploads/2021/02/Een-verjaardag-zonder-jarige-Uit-Zenit.pdf.
(4) NOS Nieuws. (2011, 11 May). Monument voor Radio Kootwijk onthuld. https://nos.nl/artikel/239671-monument-voor-radio-kootwijk-onthuld.
(5) Hartsuijker, A. & Roos, M. (2021). With the 21 cm hydrogen line from Kootwijk to Dwingeloo. https://www.camras.nl/en/blog/2021/with-the-21-cm-hydrogen-line-from-kootwijk-to-dwingeloo/.
(6) Museum Deelen. Würzburg Riese. https://www.museumdeelen.nl/radar-wurzburg-riese/.